1) Field of the Invention
The present invention relates to a head control method, a head control apparatus, and a disk-drive apparatus that control a retraction of a head that reads data from or writes data to a recording medium, whose spinning speed is controlled by a clock signal. More particularly, the present invention relates to a head control method, a head control apparatus, and a disk-drive apparatus that control a safe retraction of a data reading/writing head when an error occurs in the clock signal.
2) Description of the Related Art
Conventionally, when writing data to or reading data from a magnetic disk recording medium, such as a hard disk, a data reading/writing head is slightly raised from the surface of the magnetic disk due to a flow of air on the surface of the magnetic disk caused by the rapid spinning of the magnetic disk.
However, if the spinning speed of the magnetic disk slows down when data reading or writing is taking place, the flow of air becomes feeble. The feeble airflow is not enough to sustain the raised position of the head and causes the head to touch or stick fast to the magnetic disk due to suction, damaging the magnetic disk and causing fatal damage to the hard disk apparatus and the data stored in the hard disk apparatus.
A disk-drive apparatus is disclosed in Japanese Patent Laid-Open Publication No. H11-16259 and another in U.S. Pat. No. 6,476,996 which monitors the spinning speed of the magnetic disk and safely retracts the head when any abnormality occurs in the spinning speed.
However, the conventional disk-drive apparatuses disclosed in Japanese Patent Laid-Open Publication No. H11-16259 and U.S. Pat. No. 6,476,996 measure the spinning speed of the magnetic disk by referring to the cyclic clock signal generated by a clock circuit. However, if an error occurs in the clock signal, it leads to incorrect calculation of the spinning speed, consequently affecting the timely retraction of the head.
An error in the clock signal may occur, for instance, due to the attributes of a phase locked loop (PLL) circuit incorporated in the clock circuit. The PLL circuit is a high impedance analog circuit and is vulnerable to static electricity as compared to circuits using other logic. Consequently, an error of the clock signal can easily occur due to static electricity noise. Thus, when subjected to static electricity noise, the clock circuit generates an erroneous clock signal.
Since the control of the spinning speed of the magnetic disk is performed based on the frequency of the clock signal, an error in the clock signal leads to decreased spinning speed of the magnetic disk, causing the head to touch or stick fast to the magnetic disk due to suction, leading to fatal damage to the hard disk apparatus and the data stored in the hard disk apparatus.
Thus, a technology is needed that enables a safe retraction of the head when an error occurs in the clock signal.